The HQET Lagrangian

Abstract

In this chapter, we introduce HQET -- an effective field theory approximating QCD for problems with a single heavy quark in certain kinematics, to the leading order in $1/m$. The rest of the chapter is about methods used for calculation of one- and two-loop Feynman diagrams in HQET. These methods have much in common with the ones used to calculate loop diagrams in massless theories, such as QCD with light quarks. Therefore, for the convenience of readers, we shall recall some well-known facts about massless diagrams, too.